Java 8 List into Map

Question

I want to translate a List of objects into a Map using Java 8\'s streams and lambdas.

This is how I would write it in Java 7 and below.

private Map&l         


        

Answer 1

If every new value for the same key name has to be overridden:

public Map < String, Choice > convertListToMap(List < Choice > choices) {
    return choices.stream()
        .collect(Collectors.toMap(Choice::getName,
            Function.identity(),
            (oldValue, newValue) - > newValue));
}

If all choices have to be grouped in a list for a name:

public Map < String, Choice > convertListToMap(List < Choice > choices) {
    return choices.stream().collect(Collectors.groupingBy(Choice::getName));
}

Answer 2

I was trying to do this and found that, using the answers above, when using Functions.identity() for the key to the Map, then I had issues with using a local method like this::localMethodName to actually work because of typing issues.

Functions.identity() actually does something to the typing in this case so the method would only work by returning Object and accepting a param of Object

To solve this, I ended up ditching Functions.identity() and using s->s instead.

So my code, in my case to list all directories inside a directory, and for each one use the name of the directory as the key to the map and then call a method with the directory name and return a collection of items, looks like:

Map<String, Collection<ItemType>> items = Arrays.stream(itemFilesDir.listFiles(File::isDirectory))
.map(File::getName)
.collect(Collectors.toMap(s->s, this::retrieveBrandItems));

Answer 3

One more option in simple way

Map<String,Choice> map = new HashMap<>();
choices.forEach(e->map.put(e.getName(),e));

Answer 4

For example, if you want convert object fields to map:

Example object:

class Item{
        private String code;
        private String name;

        public Item(String code, String name) {
            this.code = code;
            this.name = name;
        }

        //getters and setters
    }

And operation convert List To Map:

List<Item> list = new ArrayList<>();
list.add(new Item("code1", "name1"));
list.add(new Item("code2", "name2"));

Map<String,String> map = list.stream()
     .collect(Collectors.toMap(Item::getCode, Item::getName));

Answer 5

Use getName() as the key and Choice itself as the value of the map:

Map<String, Choice> result =
    choices.stream().collect(Collectors.toMap(Choice::getName, c -> c));

Answer 6

Here is solution by StreamEx

StreamEx.of(choices).toMap(Choice::getName, c -> c);